1. Field of the Invention
The present invention relates to write-once-read-many (WORM) optical recording media. More specifically, it relates to write-once-read-many optical recording media on which information can be recorded at a high density even at blue-laser wavelengths and to processes for recording and/or reproducing information on the write-once-read-many optical recording media.
2. Description of the Related Art    1. Write-once-read-many Optical Recording Media Sensitive to Blue-laser Wavelengths or Shorter
With an increasing development of blue laser capable of recording of information at a very high density, write-once-read-many optical recording media sensitive to blue-laser wavelengths have been increasingly developed.
In conventional write-once-read-many optical recording media, laser beams are irradiated to a recording layer comprising an organic material to change the refractive index typically due to the decomposition and degeneration of the organic material, and thus recording pits are formed. The optical constant and decomposition behavior of the organic material used in the recording layer play an important role to form satisfactory recording pits.
For use in a recording layer of write-once-read-many optical recording media sensitive to blue-laser wavelengths, an organic material must have suitable optical properties and decomposition behavior with respect to light at blue-laser wavelengths. More specifically, the wavelengths at which recording and reproduction is performed (hereinafter briefly referred to as “recording-reproducing wavelengths”) are set at a tail on the longer-wavelength side of a major absorption band to increase the reflectance in unrecorded areas and to increase the change in refractive index caused by the decomposition of the organic material upon irradiation of laser to thereby yield a higher degree of modulation. This is because wavelengths at the tail on the longer-wavelength side of a major absorption band of such an organic material yield an appropriate absorption coefficient and a high refractive index. With reference to FIG. 1, a conventional write-once-read-many optical recording medium using an organic material in its recording layer has recording-reproducing wavelengths in the diagonally shaded area in FIG. 1.
However, no organic material having optical properties with respect to light at blue-laser wavelengths equivalent to those of conventional materials has not yet been found. To produce such an organic material having an absorption band in the vicinity of blue-laser wavelengths, the molecular skeleton must be downsized or the conjugate system must be shortened. However, this invites a decreased absorption coefficient and a decreased refractive index.
More specifically, there are many organic materials having an absorption band in the vicinity of blue-laser wavelengths, but they do not have a sufficiently high refractive index and fail to yield a high degree of modulation.
On conventional write-once-read-many optical recording media, information is recorded by the mechanism of deformation of a substrate as well as by change in refractive index due to decomposition and deformation of the organic material. For example, FIG. 3 illustrates a recorded area 101 on a substrate of a commercially available DVD-R medium observed by an atomic force microscope (AFM), showing that the substrate 105 deforms toward the reflective layer 103, which deformation leads to modulation
Examples of organic materials sensitive to blue-laser wavelengths can be found in Japanese Patent Application Laid-Open (JP-A) No. 2001-181524, No. 2001-158865, No. 2000-343824, No. 2000-343825, and No. 2000-335110.
However, these publications only teach the spectra of a solution of an organic material and a thin film prepared therefrom in their examples and fail to teach recording and/or reproducing information using the materials.
JP-A No. 11-221964, No. 11-334206, and No. 2000-43423 mention recording using an organic material in their examples but information is recorded at a wavelength of 488 nm. They only describe that satisfactory recording pits are formed and fail to teach the recording conditions and recording densities.
JP-A No. 11-58955 mention recording using an organic material in the examples but information is recorded at a wavelength of 430 nm. It only describes that a satisfactory degree of modulation is obtained and fails to teach the recording conditions and recording densities.
JP-A No. 2001-39034, No. 2000-149320, No. 2000-113504, No. 2000-108513, No. 2000-222772, No. 2000-218940, No. 2000-222771, No. 2000-158818, No. 2000-280621, and No. 2000-280620 mention recording using an organic material at a wavelength of 430 nm and a numerical aperture NA of 0.65. However, the information is recorded at a low recording density in terms of a minimum pit of 0.4 μm, equivalent to that in DVD media.
JP-A No. 2001-146074 describes recording and/or reproducing at a wavelength of 405 to 408 nm but fails to teach a specific recording density. The recording herein is performed at a low density in which 14T-FEM signals are recorded.
The optical constants of the organic materials disclosed in the above publications at wavelengths around 405 nm, which is the center emitting wavelength of blue semiconductor laser now in practical use, are not equivalent to the required optical constant for recording layers of conventional write-once-read-many optical recording media. The publications fail to disclose examples in which information is recorded at a wavelength around 405 nm at a recording density higher than that in DVD media under specific conditions and fail to teach whether or not information can be recorded at a high density of 15 to 25 GB. In addition, most of the examples in the publications are performed using media of conventional configuration comprising a substrate, an organic material layer and a reflective layer, and colorants to be used therein must have optical properties and functions the same as conventional equivalents.
Such conventional write-once-read-many optical recording media can use only organic materials having a high refractive index and a relatively low absorption coefficient of about 0.05 to about 0.10 for ensuring a satisfactorily high degree of modulation and reflectance.
These conventional write-once-read-many optical recording media using organic materials have a major absorption band in the vicinity of the recording-reproducing wavelengths, thereby have a significantly varying optical constant depending on the wavelength, namely, show a large dependency on wavelength of the optical constant as shown in FIG. 2. They significantly change their recording properties such as recording sensitivity, degree of modulation, jitter and error rate as well as reflectance with varying recording-reproducing wavelengths due to individual difference of the laser or a varying environmental temperature.
The organic materials have insufficient absorptivity to the recording light, the thickness of the resulting organic layer cannot be reduced so much, and a substrate having deep grooves must be used. In this connection, a layer of an organic material is generally formed by spin coating, and such deep grooves are filled with the organic material to form a thick layer of the organic material. Such a substrate having deep grooves is difficult to prepare, and the resulting optical recording medium may have deteriorated quality.
In addition, such a large thickness of the organic material layer leads to a narrow recording power margin and other margins in recording-reproducing properties.
Examples of techniques on layer configurations and recording processes different from those of conventional CD media and DVD media can be found as follows.
JP-A No. 07-304258 discloses a technique for recording information on a medium comprising a substrate, a layer containing a saturable absorption colorant and a reflective layer in this order based on the change of the extinction coefficient (the “absorption coefficient” as used in the present invention) of the saturable absorption colorant.
JP-A No. 08-83439 discloses a technique for recording information on a medium comprising a substrate, a metal deposition layer, a light-absorptive layer and a protective sheet arranged in this order based on the discoloration or deformation of the metal deposition layer by action of heat generated from the light-absorptive layer.
JP-A No. 08-138245 discloses a technique for recording information on a medium comprising a substrate, a dielectric layer, a recording layer containing a photoabsorption material, and a reflective layer arranged in this order by changing the thickness of the recording layer to thereby change the thickness of the grooves.
JP-A No. 08-297838 discloses a technique for recording information on a medium comprising a substrate, a recording layer containing a photoabsorption material and a metal reflective layer arranged in this order by changing the thickness of the recording layer by a factor of 10 percent to 30 percent.
JP-A No. 09-198714 discloses a technique for recording information on a medium comprising a substrate, a recording layer containing an organic colorant, a metal reflective layer and a protective layer arranged in this order by increasing the groove width of the substrate broader than an unrecorded area by a factor of 20 percent to 40 percent.
Japanese Patent (JP-B) No. 2506374 discloses a technique for recording information on a medium comprising a substrate, an interlayer, and a metal thin film arranged in this order by deforming the metal thin layer to thereby form bubbles.
JP-B No. 2591939 discloses a technique for recording information on a medium comprising a substrate, a light-absorptive layer, an auxiliary recording layer and an optical reflective layer by deforming the auxiliary recording layer to a concave shape and deforming the optical reflective layer to a concave shape along the deformation of the auxiliary recording layer.
JP-B No. 2591940 discloses a technique for recording information on a medium comprising a substrate, a light-absorptive layer, a porous auxiliary recording layer and an optical reflective layer or comprising a substrate, a porous auxiliary recording layer, a light-absorptive layer and a reflective layer by deforming the auxiliary recording layer to a concave shape and deforming the reflective layer to a concave shape along the deformation of the auxiliary recording layer.
JP-B No. 2591941 discloses a technique for recording information on a medium comprising a substrate, a porous light-absorptive layer, and a reflective layer arranged in this order by deforming the light-absorptive layer to a concave shape and deforming the reflective layer to a concave shape along the deformation of the auxiliary recording layer.
JP-B No. 2982925 discloses a technique for recording information on a medium comprising a substrate, a recording layer containing an organic colorant and an auxiliary recording layer arranged in this order by allowing the auxiliary recording layer to mix with the organic colorant to thereby shift the absorption spectrum of the organic colorant to a shorter wavelength.
JP-A No. 09-265660 discloses a technique for recording information on a medium comprising a substrate, a multi-function layer having functions as a reflective layer and a recording layer, and a protective layer arranged in this order by forming a bump between the substrate and the multi-function layer. The publication specifies metals such as nickel, chromium and titanium and alloys of these metals as the material for the multi-function layer.
JP-A No. 10-134415 discloses a technique for recording information on a medium comprising a substrate, a metal thin layer, a deformable buffer layer, a reflective layer and a protective layer arranged in this order by deforming the substrate and the metal thin layer and reducing the thickness of the buffer layer in the deformed portion. The publication specifies metals such as nickel, chromium and titanium and alloys of these metals as the material for the metal thin layer and describes that a resin which is deformable and has an appropriate flowability is used in the buffer layer, which buffer layer may further comprise a colorant to accelerate the deformation.
JP-A No. 11-306591 discloses a technique for recording information on a medium comprising a substrate, a metal thin layer, a buffer layer and a reflective layer arranged in this order by deforming the substrate and the metal thin layer and changing the thickness and optical constant of the buffer layer in the deformed portion. The publication describes that a metal such as nickel, chromium or titanium or an alloy thereof is preferably used in the metal thin layer and that the buffer layer comprises a mixture of a colorant and an organic polymer, which colorant has a large absorption band in the vicinity of the recording-reproducing wavelengths.
JP-A No. 10-124926 discloses a technique for recording information on a medium comprising a substrate, a metal recording layer, a buffer layer and a reflective layer arranged in this order by deforming the substrate and the metal recording layer and changing the thickness and optical constant of the buffer layer in the deformed portion. The publication describes that a metal such as nickel, chromium or titanium or an alloy thereof is preferably used in the metal recording layer and that the buffer layer comprises a mixture of a colorant and a resin, which colorant has a great absorption band in the vicinity of the recording-reproducing wavelengths.
These conventional techniques do not intend to provide optical recording media sensitive to blue-laser wavelengths and do not teach layer configurations and recording processes usable at blue-laser wavelengths. In addition, according to the conventional techniques, the colorant in the recording layer must be capable of absorbing light and must have a major absorption band in the vicinity of the recording-reproducing wavelengths, thus the types of colorants to be used are severely restricted.
Most of the conventional techniques record information typically by mechanism of deformation. If information is recorded mainly by mechanism of deformation, the interference among recording marks increases and thereby margins in recording and/or reproducing properties decrease, even though a satisfactory low jitter and a high degree of modulation are obtained.
As a write-once-read-many optical recording medium according to “diffusion system”, for example, TDK Corporation has announced a medium having a configuration of a substrate, ZnS—SiO2, Si, Cu, ZnS—SiO2 and Ag arranged in this order in CEATEC JAPAN 2003. The company has reported that the write-once-read-many optical recording medium has a degree of modulation of 65%, a jitter of 6% and a reflectance of 14%. An additional test made by the present inventors has revealed that when Si and Cu are arranged in adjacent layers, they gradually diffuse into another layer during storage and the recording medium shows deteriorated properties. This is a disadvantage of a medium in which components of two layers diffuse into and mix with each other. The medium requires two layers of a dielectric layer comprising ZnS—SiO2 to yield a satisfactorily high degree of modulation and requires many processes and high cost.
Thus, these conventional technologies are insufficient to provide write-once-read-many optical recording media sensitive to blue-laser wavelengths and do not teach layer configurations and recording processes usable at blue-laser wavelengths.    2. Multi-level Recordable Write-once-read-many Optical Recording Media
To increase the recording capacity, multi-level recording techniques have been developed. Recent home users generally treat large-capacity audio data and image-motion picture data, and the capacity of hard disks have increased. However, current CD or DVD optical recording media cannot provide sufficiently high recording capacities.
Under these circumstances, the ML (trademark; Multi Level) Technology has been proposed by Calimetrics, Inc. (CA) as a recording technique to increase conventional optical recording media. In short, the recording linear density is increased according to the ML Technology.
In the conventional CD or DVD optical recording media, the position or length of each recording mark edge varies corresponding to a target data message in recording, and the length of the recording mark is determined in reproduction (slice system). The current slice system will be illustrated in short below.
With reference to FIG. 4, a recording mark row (c) is initially formed on an optical recording medium using a recording waveform (b) corresponding to a target recording data (a).
Reproducing light is applied to the recording mark row (c) recorded on the optical recording medium to reproduce the information to thereby yield a reproducing signal waveform (d).
The reproducing signal waveform (d) is a dull waveform different from the recording waveform (b), a rectangular pulse, and is thereby formatted using an equalizer to yield an equalized waveform (e). More specifically, high-frequency components of the reproducing signal are amplified.
Next, the point of intersection of the equalized waveform (e) and the threshold is detected. A binary data (f) is then outputted as one (1) when the point of intersection is detected within the window and as zero (0) when it is not detected. The binary data (f) obtained by the detection of point of intersection is converted according to a non-return-to-zero (NRZ) procedure to thereby yield a decoded data (g).
In contrast, according to the multi-level recording, a mark having a reflectance at multiple levels is recorded in a fixed-length area “cell”, and the information is indicated by the multi-level reflectance. More specifically, one bit is indicated by the presence or absence of a recording mark in the conventional CD or DVD optical recording media. In contrast, recording marks are recorded at, for example, eight different levels of size and is read out as reflectance at eight different levels (FIG. 5). One recording mark indicates information corresponding to three bits, and the recording density can thereby be increased. Here, bidirectional arrow 107 indicates the size of each cell.
In the multi-level recording, the beam spot diameter of laser light in reproduction is generally larger than the cell length, and one recording mark indicates information corresponding to three bits. Thus, the recording linear density can be increased to thereby increase the recording capacity without narrowing the track pitch.
Examples of such multi-level recordable write-once-read-many optical recording media can be found in JP-A No. 2001-184647, No. 2002-25114, No. 2002-83445, No. 2002-334438, No. 2002-352428, No. 2002-352429 and No. 2002-367182. JP-A No. 2001-184647 discloses a concept of multi-level recording on an optical recording medium having a recording layer comprising an organic colorant and a concept of multi-level recording on the optical recording medium in a depth direction of the recording layer. However, this technique intends to provide a multi-level recordable write-once-read-many optical recording medium sensitive to red laser wavelengths, whose layer configuration and organic colorant used are the same as those of conventional CD or DVD write-once-read-many optical recording media.
Aforementioned JP-A No. 2002-25114 discloses a multi-level recordable optical recording medium including a substrate and a recording layer of an organic colorant, which substrate has a specific glass transition point, reflectance and thermal conductivity.
Aforementioned JP-A No. 2002-83445 discloses a multi-level recordable optical recording medium including a recording layer comprising an organic colorant, which organic colorant has specific thermal decomposition properties.
Aforementioned JP-A No. 2002-334438 and No. 2002-352428 each disclose a multi-level recordable optical recording medium having a recording layer comprising phthalocyanine or cyanine colorant, in which the relationships among the wavelength, numeral aperture NA and groove width are specified.
Aforementioned JP-A No. 2002-352429 discloses a multi-level recordable optical recording medium having a recording layer comprising an organic colorant, in which the relationship between the thickness of the recording layer on a groove and the groove depth is specified.
Aforementioned JP-A No. 2002-367182 discloses a multi-level recordable optical recording medium having a recording layer comprising an organic colorant, in which the reflectance in an unrecorded area is specified within a range of 40% to 80%.
To record information at a higher density, the cell length in the multi-level recording must be reduced to the same level as the minimum mark length in the conventional binary recording. Namely, the minimum mark in the multi-level recording is much shorter (smaller) than the minimum mark in the binary recording.
If multi-level recording can be performed at a sufficiently high density using a conventional recording material with a conventional layer configuration, this means that the minimum mark would be shortened even using the conventional recording material with a conventional layer configuration and means that the minimum mark length could be reduced in the binary recording and information could be recorded at a higher density. However, the recording density in the conventional binary recording technique cannot be actually increased unless a special recording-reproducing system is employed.
To provide multi-level recordable write-once-read-many optical recording media which are recordable at a higher density than conventional equivalents according to the binary recording, novel recording materials and layer configuration must be developed.
However, the aforementioned conventional technologies employ conventional recording materials and layer configurations in multi-level recording, although some of conditions such as the thickness of the recording layer and the material of the reflective layer are slightly modified. They cannot form shorter recording marks than conventional equivalents and cannot record and reproduce recording marks much smaller than conventional equivalents with a higher reliability. In short, they only achieve the reproduction of a somewhat smaller recording mark with good reliability by action of the recording and reproducing techniques, and simply apply the recording and reproducing techniques to write-once-read-many optical recording media.
In addition, the conventional techniques form recording marks typically by means of deformation (FIG. 3). The deformation presents no problem when the pitch between recording marks is sufficiently long, i.e., the recording linear density is low, or when the length of a cell in which a multi-level record is formed is not longer than the beam diameter of the reproducing light. However, the deformations interfere with each other and the interference becomes nonlinear when the recording linear density is high or when the length of a cell in which a multi-level record is formed is longer than the beam diameter of the reproducing light.
The phrase “the interference is linear” means that the deformation as a result of interference has a shape substantially indicated by the sum of the deformation in a cell and the deformation of an adjacent cell. FIGS. 6A, 6B and 6C are a plan view, a sectional view, and a sectional view as a sum, respectively of three recording marks which are formed in successive three cells mainly by means of deformation without interference.
FIGS. 7O, 7A, 7B, 7C, 7D and 7E schematically illustrate a reproducing signal which varies depending on the difference of interference among the deformations in three cells. In this case, three recording marks are formed in successive three cells mainly by means of deformation and the total length of the three recorded cells is smaller than the diameter 109 of reproducing beam. If the interference in deformation is linear, the resulting deformation is as shown in FIG. 7B. However, if the interference in deformation is not linear, the resulting deformation is modified as shown in FIG. 7C or FIG. 7D.
The interfered deformation has a length smaller than the diameter 109 of reproducing beam, and the difference in the deformation is therefore not detected. Accordingly, a reproducing signal as shown in FIG. 7E can be substantially obtained even when the deformation varies as shown in FIGS. 7B, 7C and 7D.
Exact data can therefore be decoded by detecting the reflection levels at sampling times T1, T2 and T3 shown in FIG. 7E.
FIGS. 8O, 8A, 8B, 8C, 8D, 8E, 8F and 8G schematically illustrate the relationship between the interference in deformation and the reproducing signal when successive seven recording marks mainly based on deformation are formed in successive seven cells and the total length of the cells is larger than the diameter 109 of reproducing beam.
The interference in deformation in this case becomes more nonlinear than the case shown in FIGS. 7B, 7C and 7D, and the interfered deformation is as shown in FIGS. 8B, 8C and 8D when simply illustrated. The interfered deformations each have a length larger than the diameter 109 of reproducing beam, and the difference among the deformations can be clearly detected. Thus, reproducing signals shown in FIGS. 8E, 8F and 8G are obtained from the deformations in FIGS. 8B, 8C and 8D, respectively.
Accordingly, when reflection levels are detected at sampling times T1 through T7 shown in FIGS. 8E, 8F and 8G, different data corresponding to the different deformations are decoded, thus failing to decode the exact data.
As is described above, recording of data mainly based on deformation leads to different interference behaviors among recording marks depending on recording patters, and the resulting reproducing signals cannot be predicted. Thus, the data are not recorded and/or reproduced properly at a higher density.    3. Recording-reproducing Technique Using PRML system
As another possible solution to achieve high-density recording than the ML recording technique, the application of partial response and maximum likelihood (PRML) technique to optical recording media has been studied.
With an increasing recording linear density to achieve high-density recording, the reproducing signal has a dull waveform. In other words, with reference to FIG. 4, the reproducing signal waveform (d) is not a rectangular waveform as in the recording waveform (b). The high frequency components of the reproducing signal are amplified using an equalizer and the reproducing signal is converted to have an equalized waveform. When the reproducing signal has a dull waveform with an increasing density, a larger quantity of high frequency components must be amplified. In amplification of the high frequency components, signal degrading components are also amplified by the equalizer, thus inviting significantly decreased signal-to-noise ratio (SNR) of the reproducing signal. PRML is a reproducing signal processing system to prevent SNR of the reproducing signal from decreasing even in high density recording.
The PRML system will be briefly illustrated below.
FIG. 9 illustrates a recording data (a) as target information, a recording waveform (b), a recording mark row (c), a reproducing signal waveform (d) and equalized waveforms (e), (f) and (g).
More specifically, the equalized waveforms (e), (f) and (g) are obtained as a result of equalization of the reproducing waveform (d) by an equalizer depending on PR(1,1) characteristic, PR(1,2,1) characteristic and PR(1,2,2,1) characteristic, respectively. The PR(1,1) characteristic is a characteristic in which an impulse response appears at the rate of 1:1 at two successive identification points. The PR(1,2,1) characteristic is a characteristic in which an impulse response appears at the rate of 1:2:1 at three successive identification points. The PR(1,2,2,1) characteristic is a characteristic in which an impulse response appears at the rate of 1:2:2:1 at four successive identification points. The equalized waveforms (e), (f) and (g) in FIG. 9 show that an equalized waveform becomes duller with an increasing complexity of the PR characteristic.
In the PRML system, an increase in the signal degrading component in the equalizer can be suppressed by equalizing the reproduced waveform into a waveform of a PR characteristic which is closer to the characteristic of the reproduced waveform.
In the reproduction signal processing of PRML system, a Viterbi decoder which is a representative one of maximum likelihood decoders is generally used as a most-likelihood decoder in decoding of the equalized waveform signals. For example, if the reproduced waveform is equalized into a waveform of the PR(1,2,1) characteristic by the equalizer, the Viterbi decoder selects a series having the smallest error with respect to the sample series of the equalized waveform from all of the reproduced waveform series which satisfy the PR(1,2,1) characteristic and estimates and outputs recording data (binary data, decoded data) used as a source for generating the selected reproduced waveform series.
Thus, the PRML system realizes high-density recording even using a conventional optical system. However, even the PRML system cannot record and reproduce information with high reliability when the interference among recording marks (intersymbol interference) becomes large and becomes nonlinear, namely, when a predictable interference among recording marks occurs. In other word, the PRML system can be applied onto to such a case in which a predictable interference among recording marks occurs. If an interference among recording marks different from the predicted one occurs, the advantages of the PRML system are not obtained.
The deformation of recording marks must be prevented to suppress the interference among recording marks at a predictable level.
By providing write-once-read-many optical recording media on which information can be recorded by multi-level recording at a wavelength of blue-laser wavelengths or shorter, recording marks with higher quality than those obtained by the conventional binary recording technique can be formed. Information can be recorded on the resulting write-once-read-many optical recording media by the conventional binary recording technique at a wavelength of blue-laser wavelengths or shorter and also by multi-level recording at a higher density by the application of the PRML system. Requirements to achieve write-once-read-many optical recording media sensitive to blue-laser wavelengths or shorter wavelengths can be considered as requirements to provide write-once-read-many optical recording media that allow multi-level recording at a wavelength of blue-laser wavelengths or shorter.
The requirements to write-once-read-many optical recording media that allow multi-level recording at a wavelength of blue-laser wavelengths or shorter are the following requirements (1), (2) and (3):    (1) smaller recording marks;    (2) less interference among recording marks; and    (3) higher stability of recording marks.
In most of the conventional write-once-read-many optical recording media, information is recorded mainly based on deformation as described above.
In binary recording, the minimum mark has a sufficient size with respect to the diameter of reproducing beam (approximately half the diameter of reproducing beam), the amplitude derived from the minimum mark is sufficiently large, and the deformation in the minimum mark is large.
In contrast, in multi-level recording, the minimum mark has an insufficient size with respect to the diameter of reproducing beam, and the amplitude derived from the minimum mark in multi-level recording is approximately one-half to one-tenths or less the amplitude derived from the minimum mark in binary recording, and the deformation in the minimum mark is very small.
Conventional CD or DVD write-once-read-many optical recording media have a layer of an organic colorant having optical absorptivity arranged directly adjacent to a substrate. Thus, the substrate deforms to a large extent. The degree of modulation is primarily affected by the deformation of substrate and is secondarily affected by the decomposition of the organic colorant. A deformation of the substrate within an elastic deformation region may be relieved, for example, by extraneous heat. A deformation of the substrate exceeding the elastic deformation region is limited, but its shape may significantly vary with the heat applied upon the formation of an adjacent recording mark or with the deformation of the adjacent recording mark.
FIGS. 10 and 11 illustrate these phenomena.
FIG. 10 shows recording marks in a write-once-read-many optical recording medium having a conventional structure of a substrate, a colorant layer, an Ag reflective layer and a protective layer arranged in this order.
FIG. 10 illustrates a waveform A of a reproducing signal; an atomic force micrographic (AFM) image B of the surface of the substrate after removing the protective layer, Ag reflective layer and colorant layer; and a deformation C of the cross section of the substrate as determined based on the AFM image B. FIG. 10 shows that the recorded area is much largely deformed with a concave shape at a center part of the recording mark. The interference in deformation (interference within one recording mark) is nonlinear, as illustrated in FIGS. 7C, 7D, 8C and 8D.
FIG. 11 illustrates recording marks obtained by recording the information as in FIG. 10 on the conventional write-once-read-many optical recording medium and applying a weak direct-current light of about one-fifths of the recording power to the medium.
FIG. 11 illustrates a waveform A of a reproducing signal; an atomic force micrographic (AFM) image B of the surface of the substrate after removing the protective layer, Ag reflective layer and colorant layer; and a deformation C of the cross section of the substrate as determined based on the AFM image B. FIG. 11 shows that the deformation of substrate changes and thus the waveform of the reproducing signal changes upon irradiation of the weak direct-current light. This is probably because the application of the weak direct-current light relieves the strain in the deformed portion of the substrate.
The fact that the shape of deformed portion of the substrate varies upon irradiation of such a weak direct-current light shows that the colorant layer on the recording mark should have a sufficient optical absorptivity and that the conventional write-once-read-many optical recording medium generates the degree of modulation mainly based on deformation.
Recording mainly based on deformation invites the following problems:    (1) the interference in deformation within one recording mark increases, and the waveform of the reproducing signal varies depending on the deformation, i. e., depending on the recording mark length;    (2) the interference in deformation among recording marks increases, and the waveform of the reproducing signal varies depending on the deformation, i. e., depending on the recording pattern such as the types of recording marks between anterior and posterior tracks or between adjacent tracks; and    (3) the deformation is relieved in reproduction, in recording onto an adjacent track, in leaving at high temperatures or in leaving for a long period of time, and the waveform of the reproducing signal varies.
These problems invite the following disadvantages:    (a) deteriorated jitter, error rate and other properties;    (b) narrowed recording power margins in jitter, error rate and other properties;    (c) unreasonable asymmetry largely shifted from zero under recording conditions to yield the optimum jitter or minimum error rate;    (d) unstable formation of small recording marks; and    (e) unpredictable interference among recording marks.
These disadvantages and problems also occur in conventional binary recording but are significant in write-once-read-many optical recording media for recording at a higher density, i.e., write-once-read-many optical recording media corresponding to the multi-level recording and/or PRML system.
In addition, the conventional write-once-read-many optical recording media each having a recording layer comprising an organic material have the following disadvantages (i), (ii), (iii) and (iv):    (i) very narrow or small degree of freedom in selection of the organic material;    (ii) very large dependency on wavelength;    (iii) deep grooves of the substrate for satisfactory recording-reproducing properties; and    (iv) no recording in “lands” between grooves.